The present invention relates to an apparatus for measuring torque in rotating shafts. The apparatus provides high sensitivity to torsional changes yet is sufficiently strong to withstand high input forces. Particularly, the invention is a non-contacting, inductively coupled, apparatus for sensing torque induced leakage flux changes in a rotating ferromagnetic rod or shaft, in which a lost motion spline provides good resolution and linearity to low input forces, yet also enables the torque sensor to withstand high input torque. The invention has particular application in the measurment of torque in a power steering system of an automotive vehicle.
Automobile power steering systems have traditionally used hydraulic servo-valves and hydraulic pumps to provide drivers with the necessary power assistance for steering. Most of the systems have evolved into mechanisms which use force feedback means to provide steering "feel". One major disadvantage of hydraulic power steering devices is the need to maintain hydraulic fluid, such as oil, at high pressures. The necessary fluid pressure generally is supplied by an engine driven pump. The parasitic loss in such a pump is relatively high so that even at times of zero power assist, the pump still draws power from the engine. Such parasitic losses are becoming more important as engine sizes decline while concern for efficiency continues.
The present invention replaces the prior art hydraulic system with an electric power steering system. However, such a system suitable for measuring steering wheel input torque requires a torque sensor having both high sensitivity to small input forces and sufficient strength to withstand large input forces on the order of 100 ft.-lbs.
Several different types of torque sensors are well-known. However, these known systems are not suitable for an automotive power steering system. In the field of mechanical instrumentation, the measurement of torque in rotating shafts has always been difficult. Torque is either measured indirectly, as a function of power and speed, or measured directly. Large machines, operating at relatively high torques, and very small machines, operating at relatively low torques, preclude the use of indirect torque sensing means, thus requiring a direct measurement technique. In other areas as well, direct measurement is preferred. However, direct measurement of torsion in rotating shafts can only be accomplished by measuring the actual strain in the shaft or by measuring the reaction torque (moment) with respect to a stable reference platform, which is difficult in certain situations.
Conventional methods for the direct measurement of shaft torsion can generally be grouped into two categories: contacting and non-contacting.
Contacting methods of measuring torque, such as the application of strain gages to the strained member, are common. However, traditional strain gage torque sensors, with either rotating brushes or rotating transformers, are far too expensive for automobile use, and are probably not reliable enough over the environmental extremes encountered in normal usage. Moreover, strain gages function best when used in conjunction with stationary members. When applied to a rotating shaft, wires connecting the gages must be run through slip rings to the source of excitation, detection and amplification. Slip rings are notoriously noisy (electrically), subject to wear, and expensive to apply. Transformer coupled strain gage torque sensors are also known. However, the complexity and high cost of such rotary transformer instruments relegates their application to laboratory or test-stand environments. They are not suitable to an automotive power steering system. Umbilical connection to a shaft mounted strain gage may be used in power steering applications since rotation is not continuous but most often limited to less than four turns lock to lock. The longevity of such coil/uncoil connections is suspect, however, particularly when the requirement of automotive safety and reliability are considered.
A common non-contacting method of measuring torque generally involves measurement of magnetic properties. The change of magnetic properties of various alloys as a result of an induced stress is well-known. In particular, the permeability of a magnetic material tends to increase due to tensile stress and tends to decrease due to compressive stress. This effect has been utilized in some torque transducers. For example, in commonly assigned U.S. Pat. No. 4,414,855 the change in permeability of a magnetic layer on the surface of a non-magnetic cylindrical rod is sensed by one or more pick-up coils located adjacent to a stressed rotating rod. For a given pick-up coil, the inductance of the coil is directly proportional to the permeability of the magnetic layer. Since the permeability of the magnetic layer is directly proportional to the stress applied thereto, the inductance of the pick-up coils is directly proportional to the stress applied to the magnetic layer. Thus the stress applied to the cylindrical rod including the magnetic layer thereon may be determined by detecting the inductance of the pick-up coils. However, temperature dependence of these same magnetic properties restricts the usefulness of such non-contacting sensors. Fabrication of power transmitting shafts using such alloys also presents many difficulties.
When a metallic member is magnetized, a leakage flux is generated at any point where a discontinuity, flaw, or defect in the material exists. The quantity of leakage flux, and therefore the sensitivity of sensing devices to the defect, is dependent upon the relative orientation of the defect and the field. The present invention uses such leakage flux principles and is based upon the well known technique of eddy current testing, wherein discontinuities, cracks, inclusions or other defects in metallic objects are detected by means of changes in the flux due to induced current flow. Eddy current testing is primarily used as a sorting method or as a quality assurance tool.
Specifically, eddy currents are typically generated within an object to be inspected by induction from an adjacent coil establishing an alternating excitation current. The eddy currents then generate magnetic fields which couple to the coil at the same frequency as that of the excitation current, but which may be of a different phase. The phase and amplitude of the induced voltages depend upon the structural characteristics of the object under test. The phase relationships may be measured by appropriate signal processing circuits.
The flow of eddy currents in a test object is governed by the skin effect phenomenon. The currents decrease exponentially with depth, depending on the shape of the object, its thickness, and its electromagnetic properties. In addition to the decrease of current amplitude as depth below the surface increases, the phase angle of the current increasingly lags the excitation signal. While eddy current testing has been used in the prior art, the present invention, however, applies the eddy current testing concept in a novel manner for achieving a more useful, more reliable, more sensitive noncontacting leakage flux torque sensor which is particularly applicable to automobile power steering systems.
Another important limitation of prior art torque sensing devices which measure stress-induced material property variations in a rotating shaft, is the insensitivity of torque direction. Whether the torsional member is stressed clockwise or counterclockwise, the net induced material property change will be identical--at least in a perfectly elastic system. For many applications, such as automotive steering effort sensors, the sign (direction) of the applied torque is essential information.
In general, all electromagnetic torque transducers which rely upon reluctance or leakage flux change actually measure stress induced strain or displacement. If these displacements are relatively large for a given input stress, the transducer will be relatively sensitive. However, compliant transducers with high sensitivity are not capable of sustaining high stress safely. In order to withstand the high input torque that may be applied to a steering wheel, the transducer would have to be stiff and insensitive. One major disadvantage of prior known noncontacting leakage flux torque sensors and variable reluctance transducers is that they do not have the requisite combination of sensitivity and strength to function effectively in an automotive power steering system. Typical full scale torque input to steering wheels is on the order of .+-.70 in.-lbs. for full power assist. However, current automotive design standards require that the steering input shaft must be sufficiently strong to withstand a force of at least 100 ft.-lbs., because of the potential for power steering system failure or possible driver induced overload.
In current hydraulic servo-assisted power steering systems a compliant torsion bar is placed in series with the steering input shaft. This torsion bar is designed to deflect .+-.7.degree. for a +70 in.-lbs. torque. At higher torques, a lost motion spline arrests the excessive compliance, and permits direct drive from the steering input shaft to the steering gear box. This added torsional compliance has been introduced to provide optimal "feel" and feedback to the driver, and does much to make current power steering systems responsive and driveable.